For research and educational purposes only. Not medical advice.
Peptide half-life formula, explained
The peptide half-life formula in plain math: fraction remaining is one half to the power of time over half-life, plus near-elimination and steady-state estimates.
A half-life is the time it takes for half of a single dose to clear from the blood. Peptides and GLP-1s follow first-order elimination, which means a fixed fraction clears per unit time, not a fixed amount. That is why the math is an exponential curve, not a straight line.
This page shows the one formula that describes that curve, plus two teaching approximations built on top of it (time to near-elimination and time to steady state). Every half-life value used here is cited from an FDA label. Where a popular research peptide has no published human pharmacokinetics, the honest answer is that it has no half-life to report, and this page says so rather than inventing a number.
Formulas and worked examples
Fraction remaining after a single dose
f(t) = (1/2) ^ (t / t-half)
fraction remaining = 0.5 raised to (elapsed time / half-life)
- f(t): the fraction of a single dose still in the body after time t (1.0 is the full dose)
- t: elapsed time since the dose, in the same units as the half-life (hours here)
- t-half: the terminal half-life of the compound, in hours
Worked examples
A compound with a 24-hour half-life, 24 hours after a dose.
- 0.5 ^ (24 / 24) = 0.5 ^ 1 = 0.5.
= about 50% remaining
The same compound, 48 hours after the dose.
- 0.5 ^ (48 / 24) = 0.5 ^ 2 = 0.25.
= about 25% remaining
Semaglutide (about a 165-hour, roughly 1-week half-life), 1 week after a dose.
- 0.5 ^ (168 / 165) is very close to 0.5 ^ 1.
= about 50% of that single dose remains after a week, which is why it is dosed weekly
- Each half-life removes half of what is left, so the amount falls 100% to 50% to 25% to 12.5%, and so on.
- Half-life describes clearance from the blood. It is not the same as duration of noticeable effect, which depends on the compound and the person.
Time to near-elimination
95% cleared at about 4.32 x t-half
about 95% is cleared after 4.32 half-lives, because 0.5 raised to 4.32 is about 0.05
- t-half: the terminal half-life, in hours
Worked examples
Tirzepatide, about a 120-hour (roughly 5-day) half-life.
- 4.32 x 120 hours = about 518 hours.
- 518 hours / 24 = about 21.6 days.
= about 95% of a single dose is cleared after roughly 22 days
- The 4.32 figure is just arithmetic: it is the number of half-lives where only 5% is left.
Time to approximate steady state
steady state at about 5 x t-half
reaching a stable level on a repeated fixed-interval schedule takes about 5 half-lives
- t-half: the terminal half-life, in hours
Worked examples
Semaglutide, about a 165-hour half-life, dosed weekly.
- 5 x 165 hours = 825 hours.
- 825 hours / 24 = about 34 days.
= levels approach steady state after about 4 to 5 weeks of consistent weekly dosing
- About 5 half-lives to steady state is a standard pharmacokinetic teaching convention. The single-dose estimator on pepSmart does not model dose-to-dose accumulation; the Pro PK simulator does.
Common mistakes
- Treating half-life as duration of effect. They are related but not the same number.
- Assuming a straight-line decay. First-order elimination is exponential: it removes a percentage, not a fixed amount, each interval.
- Inventing a half-life for a peptide that has none. BPC-157 and TB-500 have no established human pharmacokinetics, so no honest curve exists for them, and they are excluded from the decay tools.
- Using an animal half-life as if it were a human one. They can differ by a lot.
Frequently asked questions
- What is the peptide half-life formula?
- The fraction of a single dose remaining after a given time equals one half raised to the power of elapsed time divided by the half-life. For a 24-hour half-life, 50% remains at 24 hours, 25% at 48 hours, and 12.5% at 72 hours.
- How long until a peptide is basically gone?
- About 95% of a single dose is cleared after roughly 4.32 half-lives, and about 99% after roughly 6.6 half-lives. For tirzepatide (about a 5-day half-life) that is roughly 22 days to 95% cleared.
- How long to reach steady state?
- On a consistent fixed-interval schedule, levels approach steady state after about 5 half-lives. For semaglutide (about a 1-week half-life) that is roughly 4 to 5 weeks of weekly dosing.
- Why does pepSmart not show a half-life for BPC-157?
- Because there is no established human pharmacokinetics for it. The evidence base is overwhelmingly animal and preclinical, so a human half-life cannot be reported honestly. Rather than fabricate a curve, BPC-157 and TB-500 are excluded from the decay tools.
Sources
- Semaglutide terminal half-life about 1 week (Wegovy and Ozempic FDA prescribing information, Novo Nordisk).
- Tirzepatide terminal half-life about 5 days (Mounjaro and Zepbound FDA prescribing information, Eli Lilly).
For research and educational purposes only. Not medical advice.